After designing the circuit structure and device location, the EMI control of the PCB becomes extremely important for the overall design. How to avoid PCB electromagnetic interference in switching power supply has become a topic of great concern to developers. In this article, Xiaobian will introduce how to control EMI through the control of component layout.
The practice of component layout has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic device. For example, if the two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and the reflection noise will be formed at the end of the transmission line; the interference caused by the inconsistency of the power supply and the ground line will cause the product to be Performance is degraded, so when designing a printed circuit board, care should be taken to use the correct method.
Each switching power supply has four current loops:
(1), power switch AC circuit;
(2) Output rectifier circuit;
(3) Input signal source current loop;
(4) Output load current loop.
The input loop charges the input capacitor through an approximately DC current. The filter capacitor acts primarily as a broadband energy storage; similarly, the output filter capacitor is also used to store the high frequency energy from the output rectifier while eliminating the DC energy of the output load loop. . Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should be connected only from the terminals of the filter capacitor to the power supply; if the connection between the input/output loop and the power switch/rectifier loop cannot be connected to the capacitor The terminals are directly connected and the AC energy is radiated from the input or output filter capacitors to the environment.
The AC circuit of the power switch AC loop and rectifier contains high amplitude trapezoidal currents. The harmonic components of these currents are very high. The frequency is much higher than the fundamental frequency of the switch. The peak amplitude can be up to 5 times the amplitude of the continuous input/output DC current. The transition time is usually It is about 50ns. These two loops are the most susceptible to electromagnetic interference, so these AC loops must be placed before other traces in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, inductors or transformers. Place them adjacently and adjust the position of the components so that the current path between them is as short as possible. The best way to establish a switching power supply layout is similar to its electrical design. The best design flow is as follows:
Place transformer
Designing the power switch current loop
Design output rectifier current loop
Control circuit connected to the AC power circuit
Design input current source loop and input filter design Output load loop and output filter According to the functional unit of the circuit, the following principles should be met when laying out all the components of the circuit :
(1) First consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is also increased; if the size is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. The optimal shape of the board is rectangular, with an aspect ratio of 3:2 or 4:3. The components at the edge of the board are generally not less than 2mm from the edge of the board.
(2) Consider the future soldering when placing the device, not too dense.
(3) Centering on the core components of each functional circuit and surrounding it. Components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components. The decoupling capacitors are as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to load and weld, and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(6) The first principle of layout is to ensure the wiring rate of the wiring. Pay attention to the connection of the flying line when moving the device, and put the devices with the connection relationship together.
(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply
The above is how to control and suppress electromagnetic interference in the PCB board by placing and arranging components. A slight leak in these steps may cause the product to be EMI unqualified, so it is very necessary to fully understand it. Friends who are experiencing such problems can collect this article as a data reserve.
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